Completion of / Regret an Action (てしまう)

We can use the te form with the group 1 verb しまう to express regret at an action, or to emphasize that an action has been completely finished.

【Verb: te Form】しまう

Expressing Regret or Accident

友達ともだちとの約束やくそくをすっかりと忘わすれてしまう。

Completely forget about an appointment with a friend.

その時ときに冷静れいせいに考かんがえていなくて、ひどいことをうっかりと言いってしまったよ。

I wasn’t thinking clearly at the time and I mindlessly said something horrible.

Note the te form is used with the present continuous state in the first clause.

家事かじをして、大事だいじなものを捨すてしまいました。

I did the house chores and accidentally threw out something important.

The word “accidentally” isn’t explicitly used in the Japanese sentence, but the use of しまう suitably expresses the nuance of regret, so it is fair to assume that you didn’t mean to throw the item away.

電車でんしゃに乗のり遅おくれてしまった。

I accidentally missed the train.

学生がくせいのうちに、ちゃんと勉強べんきょうしなければ後悔こうかいしてしまうよ。

If you don’t study hard while you’re a student you’ll come to regret it.

Regret at NOT doing something

〜てしまう can only be used to express regret at an action that has been or will be performed; it cannot be used with the negative te-form to express regret at having not done something.

パーティで話はなした女おんなの子こに電話番号でんわばんごうを聞きかないでしまった。(✘)

[I wish I had asked the girl I spoke to at the party for her phone number.]

Instead, we would use the conditional with the past tense of “good” (よかった) to say, “it would have been good if I had done x”.

パーティで話はなした女おんなの子こに電話番号でんわばんごうを聞きけばよかった。

I wish I had asked the girl I spoke to at the party for her phone number.

Expressing Completion (To finish completely)

This structure is also heavily used to express completion. In fact, this is the original, literal meaning of the grammar point—to do something thoroughly until it is completely finished. The feeling of “regret” mentioned above actually stems from this idea: the action is 100% finished and cannot be undone.

お腹なかが空あいていて、ケーキを三つみっつ食たべてしまった。

I was hungry and so I completely finished three whole cakes.

飛行機ひこうきで本ほんを３冊さんさつ読よんでしまいました。

I completely finished reading three books on the airplane.

Casual Conversation: ちゃう and じゃう

In casual conversation, you will almost always hear an abbreviated, slurred form of 〜てしまう.

• Verbs with the te-form ending in 〜て　⇒　ちゃう
• Verbs with the te-form ending in 〜で　⇒　じゃう

Conjugation Rule: Because ちゃう and じゃう end in the “u” sound, they conjugate exactly like standard Group 1 (u-verbs). Therefore, their past tense forms are ちゃった and じゃった.

どっかで財布さいふをなくしちゃったみたい。

It seems I accidentally lost my wallet somewhere.

娘むすめは毎日まいにち友達ともだちと遊あそんじゃってて、宿題しゅくだいとかは全然ぜんぜんやらないのよ。

My daughter is just hanging out with her friends every day [regretful tone], and she doesn’t do her homework at all.